1. Field of the Invention
The present invention relates to a data structure for a data processing system and more particularly, to such a structure for the type of system referred to as a word processing or office automation system.
2. Description of the Prior Art
Word Processing and office systems are primarily concerned with the generation, editing and, for example, printing and filing, of documents. Such systems usually fall into two general classes, centralized and distributed.
A centralized system may include a central processor or computer and one or more attached terminals. Data, that is, documents of various types, and routines for operating upon the documents are stored in the central processor memory. Essentially all operations upon the documents are executed in the central processor, with the terminals operating as input and output devices for the central processor.
Distributed systems are based upon a network of smaller, active units, each having memory and processing capabilities. A distributed system may include a central, shared memory unit for storing routines and data and a number of independently operating terminals. Each terminal may include a memory for storing currently active segments of routines and data and a processor for operating upon the currently active segments.
Routine and data segments are transferred between the memory unit and the terminals as required by the operations of the terminals. An exemplary distributed system is shown in U.S. Pat. No. 4,145,739, issued Mar. 20, 1979 and assigned to Wang Laboratories, Inc., the assignee of the present invention.
In any system, whether previously existing or newly designed, the memory and processing capabilities of the system are usually determined and limited by economic and practical considerations. As a result of such limitations, a recurring problem in word processing and office systems is that of implementing increasingly more sophisticated and powerful document processing systems requiring increasingly greater memory and processing capabilities within currently available system limitations. The distributed system described in U.S. Pat. No. 4,145,739 was developed in response to this problem and provided a powerful word processing capability in a system having minimal memory and processing capabilities.
The problem described above may be regarded as being comprised of two related problem areas. The first is the system control structure, that is, a structure which includes and interrelates routines for controlling the operation of the system and routines for generating and manipulating documents. Because of the above described constraints, system control structures of the prior art have either required the use of a large and powerful computer or, in smaller systems, have only allowed document processing systems of limited capabilities. The problem is essentially one of implementing the power and flexibility of a large processor and memory system within a system having limited processing and memory capacity.
A related problem is, that due to the same constraints on memory and processing capability, the control and document processing systems of the prior art have been constructed in such a manner that the system cannot be easily modified. Such modifications frequently result in sever operational problems due to unexpected or unforeseen interactions between the modified and unmodified portions of the systems.
The second problem area is that of providing a document structure having the flexibility and expansion capability to allow the generation and manipulation of very complex documents within the above described constraints on memory and processing capability.